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Acid spun carbon nanotube (CNT) fibers were investigated for their field emission properties and performance was determined to be dependent on fiber morphology. The fibers were fabricated by wet-spinning of pre-made CNTs. Fiber morphology was controlled by a fabrication method and processing conditions, as well as purity, size, and type of the CNT starting material. The internal fiber structure consisted of CNT fibrils held together by van der Waals forces. Alignment and packing density of the CNTs affects the fiber's electrical and thermal conductivity. Fibers with similar diameters and differing morphology were compared, and those composed of the most densely packed and well aligned CNTs were the best field emitters as exhibited by a lower turn-on voltage and a larger field enhancement factor. Fibers with higher electrical and thermal conductivity demonstrated higher maximum current before failure and longer lifetimes. A stable emission current at 3 mA was obtained for 10 h at a field strength of <1 V μm(-1). This stable high current operation makes these CNT fibers excellent candidates for use as low voltage electron sources for vacuum electronic devices.

Thick barium hexaferrite ͓BaO• ͑Fe 2 O 3 ͒ 6 ͔ films, having the magnetoplumbite structure ͑i.e., Ba M͒, were epitaxially grown on c-axis oriented GaN/ Al 2 O 3 substrates by pulsed laser deposition followed by liquid phase epitaxy. X-ray diffraction showed ͑0,0,2n͒ crystallographic alignment with pole figure analyses confirming epitaxial growth. High resolution transmission electron microscopy images revealed magnetoplumbite unit cells stacked with limited interfacial mixing. Saturation magnetization, 4M s , was measured for as-grown films to be 4.1Ϯ 0.3 kG with a perpendicular magnetic anisotropy field of 16Ϯ 0.3 kOe. Ferromagnetic resonance linewidth, the peak-to-peak power absorption derivative at 53 GHz, was 86 Oe. These properties will prove enabling for the integration of low loss Ba M ferrite microwave passive devices with active semiconductor circuit elements in systems-on-a-wafer architecture.

Reactive molecular-beam epitaxy of GaN layers directly on 6H-SiC(0001) Appl. Phys. Lett. 75, 944 (1999); 10.1063/1.124562Low temperature sapphire nitridation: A clue to optimize GaN layers grown by molecular beam epitaxy GaN nanocolumns of exceptional crystalline quality have been grown by molecular beam epitaxy on both silicon ͑111͒ and sapphire ͑0001͒ substrates. Reflection high energy electron diffraction produces a unique diffraction pattern for in situ verification of columnar growth. Subsequent molecular beam epitaxial overgrowth of the nanocolumns has been used to improve the quality of thin film GaN layers when compared to GaN films grown directly on sapphire substrates. Transmission electron microscopy was used to confirm the absence of threading dislocations in the selected columns. Scanning electron microscopy of overgrown material demonstrated surface morphology similar to thin films grown in the intermediate ͑Ga-rich͒ growth regime, or a pattern of densely packed hexagonal structures, depending on growth conditions. Low temperature photoluminescence ͑PL͒ spectra demonstrated a greater than two orders of magnitude improvement in PL intensity of overgrown film versus direct film growth.